Unlike in Sindarin, Quenya does form plurals by using suffixes. There are only a few, so they should be easy to remember. Which suffixes to use depends upon exactly what kind of plural is being made, and what letter the word ends with.
Here, plurals are not of nouns only. In Quenya, adjectives must agree with their nouns on number, therefore a plural noun will have a plural-form adjective. Also, verbs must agree with their subjects in number.
NOUNS (including case or pronoun endings)
-r: used when ending in -a, -i, -o, -u, -ië, or ending in -lë
-a > -ar
-i > -ir
-o > -or
-u > ur
-ië > -ier
-lë > -ler
-i: used when ending in -ë, or ending in a consonant
-ë > -i
-n > -ni (an example; this rule holds to all consonants)
-li: added on, used to denote a small number, not many; "some"
lin-: prefixed, used to denote a large number; "many" (if word begins in m-, r-, s-, prefix is respectively lim-, lir-, lis-)
-u: a dual ending meant to describe a natural pair of something; something that occurs as a pair in nature (such as legs, ears, etc), also it refers to a mated pair -- "spouses" = "veru". If the noun ends already in a vowel, the vowel is replaced by -u; if it ends in a consonant, the -u is added on.
-t: added on, a dual ending meant to describe any two things together
ADJECTIVES (including pronoun endings)
-ë: used when ending in -a
-a > -ë
-i: used when ending in -in, -ë
-in > -ini
-ë > -i
-ië: used when ending in -ëa
-ëa > -ië
VERBS (including tense or pronoun endings)
To form the plural, add an -r:
-a > -ar (an example; this rule holds with all ending letters)
-lië: "people", "folk"
-hor: host, horde (uncomplimentary term)